feat(HEL-402): 接通网站首批只读 datahub 并建立双路对比

默认全部读取/影子开关关闭，网站继续走旧 Tushare 链路；开启单项时只替换该类原料并在失败时回旧，问天保持旧路径。

Co-authored-by: Cursor <cursoragent@cursor.com>
Co-authored-by: multica-agent <github@multica.ai>

backend/data/datahub/errors.py

@@ -0,0 +1,12 @@
+from __future__ import annotations
+
+
+class DatahubError(RuntimeError):
+    def __init__(self, code: str, message: str, status: int | None = None) -> None:
+        super().__init__(message)
+        self.code = code
+        self.message = message
+        self.status = status
+
+    def __str__(self) -> str:
+        return f"{self.code}: {self.message}"